Polycrystalline diamond has greater impact resistance than single crystal diamond. This is because polycrystalline diamond is made up of randomly oriented grains which do not provide paths for cleavage crack propagation. In contrast, a single cleavage crack can rapidly propagate across a single crystal diamond. For these reasons, polycrystalline diamond is favored over single crystal diamond in many commercial applications.
Unfortunately, the impact resistance of polycrystalline diamond is still relatively low. This is due to diamond's high elastic modulus. This is a problem in some applications because polycrystalline diamond wears by micro-fracture and spalling, and not by atomic shearing.
The relative brittleness of polycrystalline diamond has been addressed in the prior art. The first commercially available polycrystalline diamond products were composite compacts comprised of a metallic backing layer bonded directly to a diamond layer, as shown in U.S. Pat. No. 3,745,623. The most common form of this composite compact comprised a planar disc of polycrystalline diamond grown directly onto a pre-cemented disc of tungsten carbide/cobalt (WC/Co) during hot pressing.
Substrate-supported polycrystalline diamond composites possess a number of limitations. First, polycrystalline diamond tool designs are limited by substrate-supported polycrystalline diamond configurations. There are many conceivable uses for polycrystalline diamond tools that are difficult or impossible to implement with a substrate-supported polycrystalline diamond composite. These uses include rotary tools like miniature grinding wheels and drills which are constructed symmetrically about a line and have working faces that are subjected to tangential forces. Although some work has been done to adapt substrate-supported polycrystalline diamond composites to such uses (see for example U.S. Pat. No. 4,218,999 which describes a rotary tool comprised of a cylinder of polycrystalline diamond grown around a core of pre-cemented carbide), rotary tools are generally not commercially possible to implement with substrate-supported polycrystalline diamond.
Second, the pre-cemented carbide substrate of a substrate-supported polycrystalline diamond composite has a higher coefficient of thermal expansion than the polycrystalline diamond of the composite. Because the bond between the diamond layer and the carbide substrate is formed when both materials are at a temperature ranging between 1500.degree. C. and 2000.degree. C., high stresses are created when the composite compact cools to ambient temperature.
Third, the diamond layer thickness in a substrate-supported polycrystalline diamond composite is limited by "bridging" of the fine diamond powder used in making the polycrystalline diamond. Bridging is a phenomenon which occurs when fine powders are pressed from multiple directions. During pressing the individual particles in the pressed fine powder tend to stack up and form arches or bridges which prevent the full pressing pressure from reaching the center of the powder compact. When 1 micron diamond powder is used to make a polycrystalline diamond body having a thickness greater than about 0.06 inches, the diamond towards the center of the piece is typically not compacted as densely as the exterior portions of the piece. This pressing density gradient can result in cracking and chipping of the polycrystalline diamond layer.
Other polycrystalline diamond composites have been described for use as wear resistant cutting elements. U.S. Pat. No. 3,850,053 discloses a method for making a cutting tool blank by placing a graphite disc in contact with cemented WC/Co and simultaneously exposing them both to diamond forming temperatures and pressures. U.S. Pat. No. 4,525,178 discloses a composite material that includes a mixture of individual diamond crystals and pieces of pre-sintered cemented carbide. The mixture is heated and pressurized to create intercrystalline bonds between diamond crystals, and chemical bonds between diamond crystals and pre-sintered cemented carbide pieces. U.S. Pat. No. 5,128,080 describes a method for making a diamond-impregnated carbide. The method comprises liquid phase sintering a green body fabricated from a WC/Co/graphite powder blend and transforming the graphite in the sintered mass to diamond using hot-pressing (high pressure/high temperature). The particle size of each of the phases in the consolidated product was in the range 0.3-100 microns. It was not possible, however, to make a composite having phases which each have a grain size less than 0.2 microns. The relatively large size of the resulting diamond particles can result in an easy crack propagation path in the composite. Moreover, volume fractions of diamond greater than 25 volume percent could not be incorporated into the blend because the carbon segregated from the WC and Co due to large differences in their densities.
Thermal expansion mismatch stresses exist between the diamond facing and the supporting WC/Co substrate in prior art composites. Such stresses can adversely affect the bonding of the diamond to the substrate, leading to spalling under typical service conditions.
Recent developments in the synthesis and consolidation of submicron-grained WC/Co powder has resulted in higher hardness and compressive strength in the fully sintered material. Utilizing such a material to produce a WC/Co/diamond composite opens new opportunities to design and manufacture a new generation of superhard tool materials.
Accordingly, there is a need for an improved triphasic composite and method for making same that substantially overcomes the problems and disadvantages of the prior art.